Process for the direct synthesis of hydrogen peroxide

The present invention relates to a bimetallic catalyst and a process for the direct synthesis of hydrogen peroxide from hydrogen and oxygen which uses said catalyst.

Hydrogen peroxide is a commercially important compound which is widely used as bleach in the textile and paper industry, as a biocide in the environmental field and in oxidation processes in the chemical industry.

Examples of these processes are those using titanium silicalite as catalysts, such as the epoxidation of olefins (EP-100,119), the ammoximation of carbonyl compounds (U.S. Pat. No. 4,794,198), the oxidation of ammonia to hydroxylamine (U.S. Pat. No. 5,320,819) and the hydroxylation of aromatic hydrocarbons (U.S. Pat. No. 4,369,783).

The industrial production of aqueous solutions of H₂O₂by means of a complex two-step process, is known. In this process a solution of an anthraquinone, such as butylanthraquinone or ethylanthraquinone, in an organic medium immiscible with water is first hydrogenated and then oxidized with air to produce H₂O₂which is subsequently extracted in aqueous phase.

This procedure is expensive owing to the high investment costs necessary for the complex production unit involved and the necessity of separating and disposing of the by-products generated during the oxidation phase and purifying and reintegrating the anthraquinone solution before being re-used.

Processes for the direct synthesis of hydrogen peroxide from H₂and O₂have been proposed in the art for over-coming these disadvantages. These processes generally use a catalytic system consisting of a noble metal, particularly metals of the platinum group or their mixtures, in the form of salts or as supported metals.

For example, U.S. Pat. Nos. 4,772,458 and 4,832,938 describe a process for the synthesis of aqueous solutions of H₂O₂using a catalyst based on palladium and/or platinum supported on carbon, in the presence of hydrogen ions and bromide ions. Quantities of acids ranging from 2.5 to 10 g/liter are generally required for obtaining high concentrations of H₂O₂.

The use of high quantities of acids creates serious problems linked to the dissolution of the active phase (metal) of the catalyst in the reaction medium, with the consequent instability of both the catalyst and hydrogen peroxide solution produced.

In addition, under these conditions, the solutions of H₂O₂produced are difficult to use owing to the high acid content. These processes also operate under critical conditions as concentrations of H₂higher than 5% (17% or over) are used with respect to the reaction mixture and consequently fall within the explosivity limit of H₂/O₂mixtures.

Patent application EP-492.064 describes a process for the synthesis of hydrogen peroxide from hydrogen and oxygen which uses a catalyst based on palladium supported on a halogenated resin, in particular a brominated styrene/divinylbenzene resin.

The reaction is carried out in water, in the presence of an acid selected from sulfuric, phosphoric or nitric acid. Operating according to this process however concentrations of H₂O₂of about 0.58% are obtained.

Patent application EP-504,741 describes a process for the synthesis of H₂O₂from hydrogen and oxygen which uses a catalyst based on palladium or platinum supported on an acid or superacid carrier selected from oxides of molybdenum, zirconium or tungsten.

Operating according to this process, hydrogen peroxide is obtained in concentrations of not more than 1%.

U.S. Pat. No. 5,320,921 describes a process for the synthesis of H₂O₂from hydrogen and oxygen which uses a catalyst based on palladium or platinum supported on a heteropolyacid made insoluble in water. The reaction is carried out in water in the presence of bromide ions (0.5 mmoles/liter of sodium bromide). Concentrations of H₂O₂equal to about 1.1% are obtained.

These processes of the known art are consequently characterized by a low productivity and selectivity of the reaction and by the production of H₂O₂solutions which are too dilute for an economic industrial exploitation.

Italian patent application MI 98-A-01843, filed by the Applicant, relates to a process for the direct synthesis of hydrogen peroxide using a catalyst based on palladium and/or platinum co-impregnated on a sulfonated activated carbon.

It has now been found that by carrying out the reaction between hydrogen and oxygen in the presence of a bimetallic catalyst prepared by dispersing in sequence and alternating the precursors of the single metal components of the catalyst on a carrier, it is possible to overcome the drawbacks of the known art described above. This preparation method also allows the activity and selectivity of the catalyst, the concentration of hydrogen peroxide or consumption of hydrogen, to be modulated in relation to the process requirements and availability of raw materials, for example low cost hydrogen.

In particular, the use of these catalysts gives the following advantages:

(i) the possibility of using extremely low quantities of halides (<10⁻⁴moles/l) and free acids (H⁺<10⁻²moles/l) in the reaction medium. This has beneficial effects on the stability of the catalytic system and H₂O₂solutions obtained, thus allowing the possibility of directly using the above solutions in oxidation processes. The presence of only traces of halides and acidity is not a disadvantage for direct use in the above reactions as the possible introduction of salts or acidity in downstream processes is minimized.

(ii) the production of hydrogen peroxide solutions in adequate concentrations for direct use and economically valid in oxidation processes, generally ranging from 2% to 10% by weight.

(iii) the possibility of carrying out the reaction under high safety conditions. In fact, below 4% by volume of hydrogen means operating well outside the explosivity range of inert H₂—O₂mixtures.

In accordance with this, the present invention relates to a bimetallic catalyst obtained by dispersing in sequence and alternating the precursors of the single metal components of the catalyst on a carrier, and a process for the synthesis of hydrogen peroxide by the direct reaction of hydrogen with oxygen, in a solvent medium containing a halogenated promoter and an acid promoter, in the presence of said catalyst.

In particular, the catalyst used for the purposes of the present invention is obtained by:

(a) preparation of the solutions or suspensions of precursors of the single metal components of the catalytic system;

(b) dispersion of the solutions or suspensions obtained in (a) in sequence on a carrier;

(c) treatment of the catalyst with a reducing agent of the metal and drying at 120-140° C. between one dispersion and another.

In the preparation of said catalyst, steps (b) and (c) can be repeated once or several times.

The metal components of the catalyst are selected from those of the platinum group. Palladium and platinum are preferably used.

In these catalysts the palladium is normally present in a quantity ranging from 0.01 to 5% by weight and the platinum in a quantity ranging from 0.01 to 1% by weight, with an atomic ratio between platinum and palladium ranging from 0.1/99.9 to 50/50.

The palladium is preferably present in a quantity ranging from 0.4 to 2% by weight and the platinum in a quantity ranging from 0.05 to 0.5% by weight, with an atomic ratio between platinum and palladium ranging from 1/99 to 30/70.

The dispersion of the active components on the carrier can be effected by means of precipitation, impregnation or adsorption starting from solutions of their salts or soluble complexes selected from acetates, halides and nitrates.

The reduction of the catalyst components to the metal state can be carried out by means of thermal and/or chemical treatment with reducing substances such as hydrogen, sodium formiate, sodium citrate, using preparative methods well known in the art.

The inert carrier can consist of activated carbon, silica, alumina, silica-alumina, zeolites and other materials well known in the art. Activated carbon is preferred for the preparation of the catalysts useful for the invention.

Activated carbons which can be used for the purposes of the invention are those with a low ash content and a surface area of at least 100 m²/g, in particular those having a surface area greater than 300 m²/g.

Sulfonated activated carbons described in Italian patent application MI 98-A-01843 can also be used for the purpose.

The carriers can be in powder, grain or pellet form, etc.

Before supporting the precursors of the single metals, the activated carbon can be subjected to treatment such as washing with distilled water or treatment with acids, bases or diluted oxidizing agents, for example acetic acid, hydrochloric acid, sodium carbonate and hydrogen peroxide.

In particular, it has been observed that the catalyst obtained by dispersing first the palladium and then the platinum on the carrier, is more active, whereas that obtained by dispersing first the platinum and then the palladium in sequence, is more selective.

The catalyst of the present invention is particularly advantageous in a process for the direct synthesis of hydrogen peroxide from hydrogen and oxygen in a solvent in the presence of a halogenated promoter and an acid promoter.

The catalyst is used in catalytic quantities generally ranging from 10⁻⁶to 10⁻²moles of total metal contained in the catalyst per liter of reaction medium.

Advantageous results are obtained using quantities of catalyst ranging from 10⁻⁴to 10⁻³moles of total metal contained in the catalyst per liter of reaction medium.

The reaction solvent can consist of water, a C₁-C₃alcohol or their mixtures.

Among C₁-C₃alcohols, methanol is preferred for the purposes of the invention. Among the mixtures, a mixture of methanol and water with a weight ratio ranging from 50/50 to 99.9/0.1, preferably from 90/10 to 99/1, is preferred.

The acid promoter can be any substance capable of generating H⁺hydrogen ions in the liquid reaction medium and is generally selected from inorganic acids such as sulfuric, phosphoric, nitric acids or from organic acids such as sulfonic acids. Sulfuric acid and phosphoric acid are preferred. The concentration of the acid generally ranges from 20 to 1000 mg per kg of solution and preferably from 50 to 500 mg per kg of solution.

The halogenated promoter can be any substance capable of generating halogen ions in the liquid reaction medium. Substances capable of generating bromide ions are preferred. These substances are generally selected from hydrobromic acid and its salts soluble in the reaction medium, for example sodium bromide, potassium bromide, sodium or ammonium bromate. Hydrobromic acid, sodium bromide and potassium bromide are particularly preferred.

The concentration of halogenated promoter generally ranges from 0.1 to 50 mg per kg of solution and preferably from 1 to 10 mg per kg of solution.

The production of hydrogen peroxide is carried out by reacting the oxygen and hydrogen in the reaction medium in the presence of the catalyst and promoters and in the presence of or without an inert gas selected from nitrogen, helium, argon. The gas is preferably nitrogen.

The molar ratio H₂/O₂in the feeding ranges from ½ to {fraction (1/100)}, preferably from ⅓ to {fraction (1/15)} and the concentration of hydrogen in the gaseous phase in contact with the liquid reaction medium is conveniently maintained at a value lower than 4.5% molar, outside the explosivity limits of the H₂/O₂/inert gas mixture.

According to an embodiment of the process of the present invention, the reaction can be carried out using air instead of pure oxygen.

The reaction is typically carried out at temperatures ranging from −5° to 90° C., preferably from 2 to 50° C. and at a total pressure higher than atmospheric pressure, preferably ranging from 50 to 300 atmospheres.

The process according to the present invention can be carried out batchwise, or, preferably, in continuous using a reactor suitable for the purpose and selected from those described in the art.

Operating under the above conditions, it is possible to produce hydrogen peroxide in safety conditions with a reaction productivity normally ranging from 30 to 200 g of H₂O₂(expressed as 100% H₂O₂) per liter of reaction medium per hour and with a molar selectivity towards the formation of H₂O₂, referring to the hydrogen used up, ranging from 60% to 90%. The hydrogen peroxide solutions thus obtained can be used directly in oxidation processes which comprise the use of H₂O₂without costly intermediate processing, such as acid and solvent removal operations.

The process of the present invention enables the reagents to be transformed into H₂O₂with high conversions and selectivities, obtaining H₂O₂solutions without acidity or containing only traces of acidity and/or salts.

The following examples, whose sole purpose is to describe the present invention in greater detail, should in no way be considered as limiting its scope.

EXAMPLE 1

Preparation of the Catalyst

a) 8 g of activated maritime pine charcoal in powder form (Ceca 2S/E) are charged into a 0.5 liter glass flask, containing 90 ml of demineralized water and 0.28 g of Na₂CO₃. The suspension is maintained at room temperature (20-25° C.), under stirring, for 10 minutes.

0.101 g of a solution of H₂PtCl₆(8% by weight of Pt) diluted in 10 ml of water are subsequently added dropwise.

The suspension is kept at room temperature for 10 minutes and is then heated in a water bath for 10 minutes to 90° C. A solution containing 0.76 g of sodium formiate in 10 ml of water is then added and the stirring is continued at 90° C. for 2 hours.

After cooling to room temperature, the suspension is filtered and the recovered charcoal is washed with distilled water until the chlorides have been eliminated and dried in an oven at 120° C. for 2 hours.

b) the carbon thus obtained is re-suspended in a solution containing 0.4 g of Na₂CO₃in 90 ml of water and then treated following the procedure described in a), but using a solution of 0.8 g of Na₂PdCl₄at 10% of Pd, instead of the solution of H₂PtCl₆.

After drying at 120° C., a catalyst containing 1% of Pd and 0.1% of Pt on activated carbon is obtained.

EXAMPLE 2

The same procedure is adopted as described in example 1, but inverting the sequence of Pt—Pd deposit.

In step a) the solution Na₂PdCl₄is used, and in step b) the solution of H₂PtCl₆, maintaining the same quantities. A catalyst containing 0.1% of Pt and 1% of Pd on activated carbon, is obtained.

EXAMPLE 3

Comparative

Preparation of a Co-Impregnated Catalyst

8 g of activated maritime pine charcoal in powder form (Ceca 2S/E) are charged into a 0.5 liter glass flask, containing 90 ml of demineralized water and 0.28 g of Na₂CO₃. The suspension is maintained at room temperature (20-25° C.), under stirring, for 10 minutes.

0.101 g of a solution of H₂PtCl₆(8% by weight of Pt) and 0.80 g of Na₂PdCl₄(10% Pd) diluted in 10 ml of water, are subsequently added dropwise, under stirring.

The suspension is kept at room temperature for 10 minutes and is then heated in a water bath for 10 minutes to 90° C. A solution containing 760 mg of sodium formiate in 10 ml of water is then added and the stirring is continued at 90° C. for 2 hours.

After cooling to room temperature, the suspension is filtered and the recovered charcoal is washed with distilled water until the chlorides have been eliminated and dried in an oven at 120° C. for 2 hours.

The end catalyst contains 1% of Pd and 0.1% of Pt, the active phase was obtained by co-impregnation of Pd and Pt.

EXAMPLE 4

Comparative

The same procedure is adopted as described in example 3, but using only the Pd salt in the preparation.

A catalyst is obtained, containing 1% of Pd on activated carbon.

EXAMPLE 5

Comparative

The same procedure is adopted as described in example 3, but using only the Pt salt in the preparation.

A catalyst is obtained, containing 0.1% of Pt on activated carbon.

EXAMPLE 6

Synthesis of H

2

O

2

A micropilot plant is used, consisting of a Hastelloy C autoclave equipped with a thermostat-regulation system, magnetic drag stirring, a regulation and control system of the pressure during the reaction, a filter for continuously removing the liquid phase containing the reaction products, a feeding system of the solvent mixture in which the reaction takes place and a series of instruments for the regulation and control of the gaseous feeds. The reaction trend is followed by continuously analyzing the hydrogen and oxygen in the feeding and at the outlet of the reactor.

The selectivity with respect to the converted hydrogen is calculated on the basis of the concentration of H₂O₂in the reaction effluent and on the basis of analysis of the H₂leaving the reactor. The concentration of H₂O₂which is formed is determined by titration with potassium permanganate.

0.6 g of catalyst prepared as described in example 1 and 200 g of methanol:water solution (95/5 by weight) containing 6 ppm of HBr and 300 ppm of H₂SO₄are charged into the reactor.

The autoclave is pressurized, without stirring, at 100 bars with a gaseous mixture consisting of 3.6% of H₂, 10% of O₂and 86.4% of N₂. The stirring is then started up to 800 revs/minute, the pressure is maintained with a continuous stream, 700 normal liters (Nl), of the same gaseous mixture, and 300 g/hour of a methanol:water solution having the composition defined above, is fed at the same time. The temperature inside the reactor is kept at 6° C. The results are indicated in Table 1.

<tables><table frame="none" colsep="0" rowsep="0"><tgroup align="left" colsep="0" rowsep="0" cols="3"><colspec colname="1" colwidth="84pt" align="center" /><colspec colname="2" colwidth="21pt" align="center" /><colspec colname="3" colwidth="112pt" align="center" /><thead><row><entry namest="1" nameend="3" rowsep="1">TABLE 1</entry></row><row><entry namest="1" nameend="3" align="center" rowsep="1" /></row><row><entry>Hrs of</entry><entry>H₂O₂</entry><entry>Molar selectivity</entry></row><row><entry>reaction</entry><entry>wt %</entry><entry>H₂O₂%</entry></row><row><entry namest="1" nameend="3" align="center" rowsep="1" /></row></thead><tbody valign="top"><row><entry> 5</entry><entry>4.7</entry><entry>83</entry></row><row><entry>10</entry><entry>4.6</entry><entry>84</entry></row><row><entry>20</entry><entry>4.6</entry><entry>84</entry></row><row><entry>50</entry><entry>4.5</entry><entry>86</entry></row><row><entry namest="1" nameend="3" align="center" rowsep="1" /></row></tbody></tgroup></table></tables>

EXAMPLE 7

Example 6 is repeated using the catalyst prepared in example 2.

The results obtained are indicated in Table 2.

<tables><table frame="none" colsep="0" rowsep="0"><tgroup align="left" colsep="0" rowsep="0" cols="3"><colspec colname="1" colwidth="84pt" align="center" /><colspec colname="2" colwidth="21pt" align="center" /><colspec colname="3" colwidth="112pt" align="center" /><thead><row><entry namest="1" nameend="3" rowsep="1">TABLE 2</entry></row><row><entry namest="1" nameend="3" align="center" rowsep="1" /></row><row><entry>Hrs of</entry><entry>H₂O₂</entry><entry>Molar selectivity</entry></row><row><entry>reaction</entry><entry>wt %</entry><entry>H₂O₂%</entry></row><row><entry namest="1" nameend="3" align="center" rowsep="1" /></row></thead><tbody valign="top"><row><entry> 5</entry><entry>6.5</entry><entry>70</entry></row><row><entry>10</entry><entry>6.6</entry><entry>71</entry></row><row><entry>20</entry><entry>6.4</entry><entry>71</entry></row><row><entry>50</entry><entry>6.6</entry><entry>72</entry></row><row><entry namest="1" nameend="3" align="center" rowsep="1" /></row></tbody></tgroup></table></tables>

EXAMPLE 8

Example 6 is repeated using a catalyst prepared as in example 2, but using a sulfonated carbon as carrier, prepared as described in example 1 of Italian patent MI 98-A-01843.

The results are indicated in Table 3.

<tables><table frame="none" colsep="0" rowsep="0"><tgroup align="left" colsep="0" rowsep="0" cols="3"><colspec colname="1" colwidth="84pt" align="center" /><colspec colname="2" colwidth="21pt" align="center" /><colspec colname="3" colwidth="112pt" align="center" /><thead><row><entry namest="1" nameend="3" rowsep="1">TABLE 3</entry></row><row><entry namest="1" nameend="3" align="center" rowsep="1" /></row><row><entry>Hrs of</entry><entry>H₂O₂</entry><entry>Molar selectivity</entry></row><row><entry>reaction</entry><entry>wt %</entry><entry>H₂O₂%</entry></row><row><entry namest="1" nameend="3" align="center" rowsep="1" /></row></thead><tbody valign="top"><row><entry> 5</entry><entry>6.2</entry><entry>73</entry></row><row><entry> 10</entry><entry>6.4</entry><entry>74</entry></row><row><entry> 20</entry><entry>6.5</entry><entry>73</entry></row><row><entry> 30</entry><entry>6.3</entry><entry>75</entry></row><row><entry> 50</entry><entry>6.6</entry><entry>76</entry></row><row><entry>100</entry><entry>6.5</entry><entry>75</entry></row><row><entry namest="1" nameend="3" align="center" rowsep="1" /></row></tbody></tgroup></table></tables>

EXAMPLE 9

Comparative

Example 6 is repeated using the catalyst prepared in example 3. The results obtained are indicated in Table 4.

<tables><table frame="none" colsep="0" rowsep="0"><tgroup align="left" colsep="0" rowsep="0" cols="3"><colspec colname="1" colwidth="84pt" align="center" /><colspec colname="2" colwidth="21pt" align="center" /><colspec colname="3" colwidth="112pt" align="center" /><thead><row><entry namest="1" nameend="3" rowsep="1">TABLE 4</entry></row><row><entry namest="1" nameend="3" align="center" rowsep="1" /></row><row><entry>Hrs of</entry><entry>H₂O₂</entry><entry>Molar selectivity</entry></row><row><entry>reaction</entry><entry>wt %</entry><entry>H₂O₂%</entry></row><row><entry namest="1" nameend="3" align="center" rowsep="1" /></row></thead><tbody valign="top"><row><entry> 5</entry><entry>4.8</entry><entry>75</entry></row><row><entry>10</entry><entry>4.9</entry><entry>76</entry></row><row><entry>20</entry><entry>4.7</entry><entry>77</entry></row><row><entry>50</entry><entry>4.6</entry><entry>77</entry></row><row><entry namest="1" nameend="3" align="center" rowsep="1" /></row></tbody></tgroup></table></tables>

EXAMPLE 10

Comparative

Example 6 is repeated using the catalyst prepared in example 4.

The results obtained are indicated in Table 5.

<tables><table frame="none" colsep="0" rowsep="0"><tgroup align="left" colsep="0" rowsep="0" cols="3"><colspec colname="1" colwidth="84pt" align="center" /><colspec colname="2" colwidth="21pt" align="center" /><colspec colname="3" colwidth="112pt" align="center" /><thead><row><entry namest="1" nameend="3" rowsep="1">TABLE 5</entry></row><row><entry namest="1" nameend="3" align="center" rowsep="1" /></row><row><entry>Hrs of</entry><entry>H₂O₂</entry><entry>Molar selectivity</entry></row><row><entry>reaction</entry><entry>wt %</entry><entry>H₂O₂%</entry></row><row><entry namest="1" nameend="3" align="center" rowsep="1" /></row></thead><tbody valign="top"><row><entry> 5</entry><entry>0.9</entry><entry>35</entry></row><row><entry>10</entry><entry>0.8</entry><entry>37</entry></row><row><entry namest="1" nameend="3" align="center" rowsep="1" /></row></tbody></tgroup></table></tables>

EXAMPLE 11

Comparative

Example 6 is repeated using the catalyst prepared in example 5.

The results obtained are indicated in Table 6.

<tables><table frame="none" colsep="0" rowsep="0"><tgroup align="left" colsep="0" rowsep="0" cols="3"><colspec colname="1" colwidth="84pt" align="center" /><colspec colname="2" colwidth="21pt" align="center" /><colspec colname="3" colwidth="112pt" align="center" /><thead><row><entry namest="1" nameend="3" rowsep="1">TABLE 6</entry></row><row><entry namest="1" nameend="3" align="center" rowsep="1" /></row><row><entry>Hrs of</entry><entry>H₂O₂</entry><entry>Molar selectivity</entry></row><row><entry>reaction</entry><entry>wt %</entry><entry>H₂O₂%</entry></row><row><entry namest="1" nameend="3" align="center" rowsep="1" /></row></thead><tbody valign="top"><row><entry> 5</entry><entry>1.7</entry><entry>22</entry></row><row><entry>10</entry><entry>1.5</entry><entry>25</entry></row><row><entry namest="1" nameend="3" align="center" rowsep="1" /></row></tbody></tgroup></table></tables>